Oxidation-sulfidation resistant articles

ABSTRACT

High-temperature nickel and cobalt base alloy articles such as turbine buckets are provided with improved oxidation and sulfidation resistance by the provision of a diffused aluminum and manganese layer in the surface of the article. The invention herein described was made in the course of work under a contract or subcontract thereunder with the Department of the Air Force.

United States Patent Lynch et a1.

[ 5] Mar. 14, 1972 Primary Examinerl-lyland Bizot Attorney-Sidney Carterand Peter P. Kozak [57] ABSTRACT High-temperature nickel and cobalt basealloy articles such as turbine buckets are provided with improvedoxidation and sulfidation resistance by the provision of a diffusedaluminum and manganese layer in the surface of the article. Theinvention herein described was made in the course of work under acontract or subcontract thereunder with the Department of the Air Force.

4 Claims, 3 Drawing Figures OXIDATION-SULFIDATION RESISTANT ARTICLESThis invention relates to metal articles such as turbine blades andnozzle guide vanes adapted for use in high temperature and corrosiveenvironments and more particularly to metal articles of theaforementioned type formed of a nickel or cobalt based alloy and havinga diffused surface layer which provides the article with superioroxidation and sulfidation re sistance.

High-temperature alloy components, such as turbine blades and nozzleguide vanes used in gas turbine engines are subjected to extendedperiods of service at elevated temperatures under variable stressconditions. When such components are formed of certain high-temperaturenickel based alloys and cobalt based alloys, they possess excellentstrength under most hightemperature service conditions. However, thedurability of these turbine buckets and nozzle guide vanes is materiallyreduced because of inadequate resistance to oxidation and sulfidation.

The term sulfidation" as used herein refers to the attack of nickel orcobalt based alloys by sulfur-bearing compounds such as sulfides andsulfates combined with chlorine. As applied to aircraft enginecomponents, the sulfur-bearing compounds and chlorine come fromcontaminated atmosphere, the engine fuel, and sea water, or from thereaction of the fuel and water.

It is known that the treatment of the nickel or cobalt based alloys bydiffusing aluminum into the surface of the metal provides the articleswith a high degree of oxidation resistance. A suitable aluminumdiffusion treatment is described in the US. Pat. No. 3,129,069 Hanink etal., assigned to the assignee of the present invention. While thediffused aluminum surface layer described in this patent provides thenickel and cobalt based alloys described therein with outstandingoxidation resistance, it does not provide the alloys with comparablesulfidation resistance.

It is, accordingly, the principal object of this invention to providearticles such as turbine blades formed of high-temperature nickel andcobalt based alloys which have both oxidation and sulfidation resistanceto an outstanding degree.

A more specific object of the invention is to provide the surface ofnickel or cobalt based alloys with a diffusion layer including bothaluminum and manganese in certain suitable proportions improvedsulfidation resistance as well as oxidation resistance is achieved.

In general, these and other objectives are accomplished by applying apowdered mixture containing aluminum and manganese as a layer over thesurface of the alloy to be diffusion treated and subjecting the alloy toa diffusion heat treatment whereby both the aluminum and manganese arediffused into the alloy surface to a depth of about 0.0004 to 0.006 inchto form complex intermetallic phases in the diffusion layer whichaffords the alloy surface with outstanding sulfidation and oxidationresistance. Preferably, the proportions of aluminum and manganese in themixture are such that based on the total aluminum and manganese content,the proportion of aluminum is about to 67 percent with the balancemanganese, on a weight basis.

Other objects and advantages will be apparent from the followingdescription of the invention, reference being had to the accompanyingdrawings in which:

FIG. 1 is an elevation view with parts broken away and in section of aturbine bucket formed of a nickel base alloy provided with analuminum-manganese diffused surface layer in accordance with theinvention.

FIG. 2 is a photomicrograph view of a high-temperature creep-resistantnickel base alloy showing the micrographic structure of the alloy nearits working surface after the aluminum-manganese diffusion treatment ofthis invention.

FIG. 3 is a photomicrographic view of the nickel base alloy shown inFIG. 2 showing the micrographic structure of the alloy near its workingsurface after it has been exposed to cyclic heating at elevatedtemperatures to determine sulfidation resistance.

As indicated above, this invention is concerned primarily with articlesof manufacture formed of high-temperature nickel or cobalt based alloyswhich are intended for use at relatively high temperatures in thevicinity of 2,000 F. and in oxidizing and sulfidizing environments. Gasturbine blades or buckets and stator vanes are examples of articles ofthis type to which this invention is particularly applicable.

In general, the nickel and cobalt based alloys to which this inventionis applicable are nickel based alloys containing 40-80 percent by weightnickel, 5-20 percent chromium, and the balance other constituents, andcobalt based alloys containing 45-70 percent cobalt, 15-30 percentchromium, and the balance other constituents. In the case of the nickelbased alloys the other constituents may include up to 10 percentmolybdenum, up to 5.5 percent titanium, up to 6.5 percent aluminum, upto 3 percent columbium, up to 9 percent tantalum, up to 13.5 percenttungsten, up to 2 percent hafnium, up to 1 percent rhenium, up to 1.5percent vanadium, up to 20 percent cobalt, up to 3 percent iron, andminor amounts of cesium, boron, zirconium, silicon, manganese, andimpurities in the form of sulfur, copper, and phosphorous. In the caseof the cobalt based alloys, the other constituents may include up to 7percent molybdenum, up to 10 percent tantalum, up to 16 percenttungsten, up to 16 percent nickel, up to 3 percent iron, minor amountsof carbon, manganese, silicon, titanium, boron, and zirconium, andimpurities in the form of iron, sulfur, phosphorous, and copper.

The following Table I contains examples of suitable high temperaturenickel and cobalt based alloys which may be satisfactorily provided witha thin aluminum an manganese diffusion layer in accordance with thisinvention, the composition being listed in by weight. Alloys Nos. 1, 2and 3 are nickel based alloys, and No. 4 is a cobalt based alloy.

TABLE I No. I No.2 No.3 No.4 Carbon 0.08-02 0.13-0.17 0.05-0.1 0.55-0.65Manganese 0.25 max. 0.20 max. 0.1 max. 0.1max

Chromium 13-15 8-10 17.514 18.5 23-245 Cobalt I max. 9-11 14.5-15.5Remainder Molybdenum 3.8-5.2 2.25-2.75 2.75-3.25

Tungsten 9-11 1.25-1.75 6.5-7.5

Iron 2.5 max. 1 max. 0.5 max. 1.5 max Titanium 0.5-1.25 1.25-1.754.75-5.25 0.15-0.25

Aluminum 5.5-6.5 5.25-5.75 2.25-2.75

Silicon 0.5 max. 0.20.max. 0.2 max. 0.4 max.

Sulfur 0.015 max. 0.015 max. 0.015 max.

Copper 0.1 max. 0.1 max. 0.5 max.

Tantalum 1.8-2.8 1.25-l.75 3-4 Boron 0.005-0015 0.01-0.02 0015-0025 0.01max.

Zirconium 0.05-0.15 0.03-0.08 0.4-0.6

Nickel Remainder Remainder Remainder 9-1 1 Another nickel base alloywhich may be advantageously treated in accordance with this invention isdisclosed in the US. Pat. No. 2,688,536 Webbere et al. This alloycomprises about 0.06-0.25 percent carbon, 13-17 percent chromium, 46percent molybdenum, 8-12 percent iron, 1.5-3 percent titanium, 2-4percent aluminum, 0.0l0.5 percent boron, and the balance substantiallyall nickel.

Referring more particularly to the drawings, there is shown in FIG. 1 aturbine bucket 10 for a gas turbine engine of the axial flow type. Inaccordance with this invention, an article such as this turbine bucket10 is formed ofa nickel base 12 of the type described above which isprovided with a surface diffusion layer 14 containing both aluminum andmanganese. For the purpose of illustration the thickness of thediffusion layer is considerably exaggerated, the actual thickness beingas described elsewhere herein. It is usually unnecessary to provide themanganese-aluminum diffusion layer over the fastening portion 16 of theturbine bucket.

In general, the diffusion layer 14 is formed by first applying a mixtureof aluminum and manganese to the surface of the base alloy 10 inparticulate form in a manner so as to provide an adherent layer thereofadjacent the alloy surface. The alloy surface and the adherent layer arethen heated to a temperature and for a time whereby the aluminum andmanganese are both simultaneously diffused into the base metal alloysurface to a depth of about 0.0004 to 0.006 inch to form the diffusionlayer 14. Preferably, the diffusion heat treatment is accomplished byfirst heating the coated article at about 1,300 F for about 1 hour andsubsequently heating the article for about 2 hours at about 2, l F. in ahydrogen environment.

Several methods for simultaneously diffusing the aluminum and manganeseinto the base metal have been found to provide substantially equivalentdiffusion surface layers which provide the article with markedlysuperior sulfidation and oxidation resistance.

ELECTROPI-IORETIC METHOD Example I In general, this method consists ofapplying a particulate coating of aluminum and manganese to the articleto be diffusion coated by suspending metal particles of suitableparticle size in an organic dielectric solvent and impressing a directcurrent of 50 to 500 volts between two electrodes, thus causing themetallic particles to uniformly deposit on the article which is one ofthe electrodes. Subsequently the coated article is heat treated todiffuse the metal particles into the article surface.

A coating bath was prepared consisting of a solution of 6 parts byweight isoproponal and 4 parts by weight nitromethane to which was added2.7 grams per liter zein and 0.2 grams per liter cobalt nitrate. Then,8.25 grams manganese powder per liter of the bath and 16.75 gramsaluminum powder per liter of the bath were added to the bath and mixedtherein to form a dispersion of the metal powder in the bath. The metalpowders were of a I to micron size.

A cast turbine blade specimen cast of the alloy No. l as shown in FIG. 1of the drawings, Table l, was first cleaned by dry honing, using No. 240grit or finer aluminum oxide abrasive. Blasting was discontinued whenthe surface color of the casting changed to a uniform light gray. It wasthen immersed in the bath and made cathodic. An inert stainless steelanode was immersed in the bath and a current of milliamperes per squareinch at 200 volts was applied across the electrodes for about 60seconds, whereby the powdered metals were caused to migrate to and bedeposited uniformly over the blade surface. Sufficient zein was alsodeposited along with the powders to act as a binder. About 0.07 gramsper square inch of the metal powders were deposited on the bladesurfaces. A thusly coated blade was heat treated for about 1 hour atl,300 F. and then for an additional 2 horus at 2,100 F. in a hydrogenenvironment. A diffusion layer 14 of about 0.003 inch in thickness wasformed as shown in FIG. 1 ofthe drawings.

A 500X magnification photomicrograph of the diffused layer was made asshown in FIG. 2 and an X-ray diffraction study of the diffusion layerwas made. The region of electron microprobe line-scan analysis showsfour distinct zones which are described as follows.

The a-b inner zone is very slightly enriched Cr, Mo, Mn, and Fe, anddepleted in Ni and Ti with respect to the normal structure of this alloyin the base material adjacent to it. Some intrusion of the matrix phasefrom the adjacent b-d zone can also be seen.

The b-d or first coating zone contains at least two phases. The matrixphase is enriched in Al, Mn, and Ti, and depleted in Cr, Mo, Fe, and Ni,as compared to the base material. This matrix appears to be the nickelrich side of the Ni Al, phase in which Mn, Cr, and Fe are substitutionalfor some of the Ni. Ti and Al, comprise the balance of the phase. Theincluded particulate material in this zone appears to be anintermetallic phase containing primarily Cr, M0, and Ni with lesseramounts of Al, Mn, and Fe. Intrusion of the inner zone material a-b intothis zone can be observed in the micrograph of FIG. 2.

The d-e or intermediate coating zone appears to be a single phase, butclose examination of the photograph indicates the possible presence ofan extremely fine (about 0.1 micron) randomly dispersed phase. Theelement distribution scans show a continued increase of aluminum andmanganese across this zone and a corresponding continued decrease ofchromium and titanium. The nickel also shows a very slight increase inconcentration across the zone. This is believed to be the intermetallicphase based on NiAl having other elements substitutional for both nickeland aluminum across the zone. The phase NiAl has a wide lattitude ofcomposition (nickel 65 to 83 percent and aluminum 35 to 17 percent byweight) explaining the variable composition across the zone.

The ei or outer coating zone comprises at least two and possibly threephases. The matrix phase, on the basis of aluminum and nickel content,appears to be nearly stoichiometric N iAl with some substitution of Mninto the lattice, probably for the Ni. The large particle (i) is aMoCrSi Mn phase. Other particles in this zone appear to be somewhatdifferent in composition, containing Nb, Ti, and/or Al, The extremelyfine particles seen near the surface side of this zone are too small tobe resolved on the probe.

Thereafter, the diffusion coated bucket was subjected to a test designedto establish the resistance of the coating to both sulfidation andoxidation. in general, the test consists in repeatedly heating andcooling a test specimen in a sulfidation and oxidation environment.

The equipment used consisted of a gas fired furnace having an internalshaft means for supporting and rotating a test specimen therein, spraynozzles for spraying a 1 percent by weight sulfate ion solution ofdeionized water using sodium sulfate as a source of sulfate ion, and anoptical pyrometer. One cycle of the test consisted in heating thespecimen to about l,900 F. and then permitting it to cool to 500 F.while spraying it with the aforementioned sulfate ion solution whilerotating it.

The diffusion coated turbine blade described above was subjected to 500cycles as above indicated. The bucket experienced a weight loss of only0.0488 grams. Visual inspection indicated virtually no corrosive effectand no cracking on the blade surface. To further test the blade forcracking, it was heated to about l,0OO F. in the atmosphere and visuallyobserved in this condition. At this temperature the metal assumes a goldor high-brown color which reveals any cracks on the blade surface. Nocracks were observed. Finally, the blade was dipped in Zyglo penetrantand inspected. No cracking was observed.

A 500x magnification photomicrograph of the diffused layer after thesulfidation-oxidation test was made as shown in FIG. 3 and an X-raydiffraction study of the diffusion layer was made. The region ofelectron microprobe line-scan analysis shows four distinct zones whichare described as follows. Referring to FIG. 3, the cone a-b appears toconsist of either a single complex oxide or a mixed multioxide. Theouter portion of the zone is Mn rich and contains some Fe. Both the Aland Cr content are quite low in this portion and the Ni content appearsto be les than a few percent. The inner part of this zone is high in Aland also contains some Cr and Mn. This appears to be a complex Al(Mn-Cr) oxide phase.

The zone b-c is very rich in Ni, and also contains Al, Mn, and Cr. Itdoes not etch like the balance of the matrix material in the diffusionregion and seems to be a zone from which the oxide forming elements (Al,Cr, Mn) have diffused, leaving behind a relatively Ni rich (about 80percent) region. This phase appears to be a solid solution of Al, Mn,and Cr in Ni.

The c-g zone is a complex one containing two or more phases dispersed ina Ni rich matrix. The matrix phase, based on the Al (about 9 percent byweight) content appears to be either the NiAl (about 30 percent byweight Al) or Ni Al (about 13 percent by weight aluminum) phases. The Ni(about 70 percent by weight) content in this phase considered alonewould indicate the phase to be NiAl. The phase is seen to containapproximately 70 percent by weight Ni, 9 percent by weight Al, and 4percent by weight Cr, with the other nominal alloying ingredientsassociated with Alloy No. 1 (Ti, Nb, Mo, Si, Fe) comprising no more than2 percent by weight of the total. By difference this means that thematrix phase contains about percent by weight manganese. Particle F" inthis zone appears to be a (Cr, Mo, Mn, Ni) intermetallic phase similarto the particulate phase found in the b-d coating zone of the as coatedspecimen shown in FIG. 2.

The gh zone like the adjacent zone is also multiphase in character. Thematrix is nearly the same as in the e-g zone, i.e., based on the Ni Alphase with substitution of Mn and Cr into the lattice. The massive phase(i) is Ni rich (about 65 percent by weight) and contains low aluminum(about 3 percent by weight), nominal chromium (about 3 percent byweight) Mo (about 4 percent by weight) and Mn (about 12 percent byweight) with the balance of the phase being composed of other alloyingelements normal to the No. 1 base alloy involved.

The h-j zone shows the extent of Mn diffused into the bulk of thematrix. It can readily be seen that the Mn is substitutional for the Niin the zone.

The entire procedure of Example No. l was repeated for Alloy No. l wherethe electrophoretic coating composition contained l6.75 grams per literof Mn powder and 8.25 grams per liter of aluminum powder. Micrographicexamination indicated a similar diffusion structure and after a similarsulfidation and oxidation test, a weight loss of 0.0028 grams wasmeasured with the surface showing no sign of cracking or corrosionfailure.

EXAMPLES Nos. 2 and 3 The procedure of Example No. 1 was carried outwith the Alloys Nos. 2 and 3 of Table I with similar results indicatingsimilar excellent sulfidation and oxidation resistance properties.

The procedure described in Example No. 1 using Alloy No. l as the basemetal was performed with different proportions of manganese and aluminumin the electrophoretic bath with results indicating that withproportions of aluminum of about 18 to 67 percent and the balancemanganese, satisfactory sulfidation and corrosion-resistant diffusioncoatings are ob tained. When the proportions of these metals areappreciably outside this range, test blades failed thesulfidation-oxidation tests. A test where the manganese content was16.75 grams per liter with no aluminum, produced a diffusion coatingthickness of 0.002 inch, the blade involved a loss of 0.2927 grains inthe sulfidation-oxidation test and inspection of the tested bladeindicated a failure. A test where the electrophoretic bath contained 25grams per liter aluminum and no manganese produced a diffusion coatingof 0.0004 inch, the blade showed a weight loss of 0.21 10 grams afterthe sulfidation-oxidation test and the blade failed. A test where thebath contained 40 grams per liter manganese and 10 grams per literaluminum developed a diffused coating thickness of 0.0004 inch, a weightloss of 0.3440 grams after the sulfidation-oxidation test and the bladefailed. A test wherein the manganese content was 33 grams per liter andthe aluminum content was 8.25 grams per liter in the coating bathdeveloped a diffused coating thickness of 0.0025 inch, involved a weightloss of 0.3341 grams after the sulfidation-oxidation test and the bladefailed.

Other methods for providing the diffusion layer 14 have been found toproduce diffusion coatings substantially equivalent to that formed bythe method described in Example 1.

PACKED DIFFUSION PROCESS In this method, the article to be diffusioncoated is packed in a retort surrounded by a suitable powdered mixtureconsisting of the metals to be diffused and an activator carrier. Thearticle is then heated in the retort and then in a hydrogen environmentto obtain a diffusion coating.

A uniformly mixed composition was prepared using on a weight basis 10percent aluminum powder, 25 percent manganese powder, 8 percent ammoniumchloride, and 57% aluminum oxide. The powders were of about a 5 micronsize. The composition was placed in a retort with a turbine bladespecimen formed of Alloy No. l to be diffusion coated immersed in thepowdered mixture. The packed retort was placed in a furnace and raisedto l,350 F. for one hour and then cooled in the retort to a handlingtemperature. The specimen was then removed from the retort and placed ina rack for the diffusion step. Diffusion was accomplished by placing therack in a hydrogen atmosphere (25 F., 2. max.) furnace, heated to about2,100 F. and held at this temperature for 2 hours. The blade was thenremoved from the furnace and allowed to cool in a draft-free closure.Metallographic examination before and after the sulfidation-oxidationtest as described in Example 1 indicated a similar diffusion coating andsimilar oxidation-sulfidation resistance characteristics.

Similar test were performed with a pack consisting of on a weight basis10 percent aluminum powder, 8 percent ammonium chloride, 34 percentferro-manganese (78 percent by weight manganese) and 48 percent aluminumoxide. Satisfactory results were also obtained using a pack consistingof on a weight basis 10 percent aluminum powder, 10 percent manganesepowder, 8 percent ammonium chloride, and 72 percent aluminum oxide.Diffusion layer thicknesses in the range of 0.001 to 0.003 inch wereobtained in these tests.

SLURRY METHOD In this method an inorganic liquid binder is blended insuitable proportions with aluminum and manganese powders and applied tothe surfaces of the article to be coated by dipping or spraying. Afterdrying the coating, the article is heat treated to obtain a diffusedcoating.

A slurry was prepared by mixing on a weight basis of one part aluminumpowder and 4.5 parts manganese powder together with about 1 part ofaqueous potassium silicate and a small amount of potassium nitrate as awetting agent to form a brushable or sprayable slurry. The powders usedwere of a 1 to 5 micron size. A turbine bucket specimen formed of AlloyNo. l was coated with the slurry by dipping it into a stirred bath ofthe slurry. The specimen was removed and air dried for thirty minutesand then oven baked for about 1 hour at 230 to form a coating bonded tothe specimen. The coated specimen was then placed in an oven having aninitial temperature of not in excess of 400 F. The furnace temperaturewas raised to l,325 F. and the specimen was subjected to thistemperature for 1 hour. The furnace temperature was then increased to 2,F. and the specimen was subjected to this temperature for 2 hours. Thefurnace was then allowed to cool to room temperature. Diffused coatingthicknesses in a range of 0.001 to 0.003 inch were obtained on thespecimen surfaces by this method. This diffusion layer was similarmetallurgically to that described in Example 1 and sulfidation-oxidationtests indicated similar results. In another test, a slurry was preparedcontaining two parts aluminum powder and one part manganese powder.Similar results were obtained.

A suitable slurry for use in this method may be prepared by mixingsuitable proportions of manganese powder with Sermetal 222, a product ofTeleflex Corp. which contains an alkali metal binder aluminum powder andalumina. Good results are obtained using, for example, 60 grams Sermetal222 to 90 grams manganese powder.

Cobalt based alloy articles are provided with similar improvement insulfidation and oxidation resistance by the same methods-described abovewith the cobalt generally serving the role of the nickel in the abovetests and with cobalt forming intermetallic constituents in thediffusion layer as does nickel in the above tests. Specific examples ofapplicable cobalt based alloys include the Alloy No. 4 of Table I aswell as the commercial alloy consisting of on a weight basis 0.50percent carbon, 0.5 percent manganese, 0.5 percent silicon, 0.02 percentsulfur, 25.5 percent chromium, 10.5 percent nickel, 7.5 percenttungsten, 1.0 percent iron, with the remainder substantially cobalt andan alloy consisting of 0.10 percent carbon, 1.5 percent manganese, 0.20percent silicon, 0.02 percent phosphorous, 0.02 percent sulfur, 20.0percent chromium, percent nickel, l5 percent tungsten, 2.0 percent iron,and the balance substantially cobalt.

The above-described tests and other tests have demonstrated thatmarkedly improved sulfidation and oxidation resistance is obtained whenthe aluminum constitutes by weight to 67 percent of thealuminum-manganese content of the materials diffused into the base metaland when the diffusion layer is present in the range of 0.0004 to 0.006inch in thickness. Optimum oxidation and sulfidation resistance havebeen achieved with the aluminum content of about 18 percent andmanganese content of 82 percent. The preferred diffusion layer thicknessis about 0.003 inch. It has been found that where the diffusion depth isgreater than about 0.006 inch, it spalls due to brittleness. Diffusionlayer thicknesses of less than 0.0004 do not provide effective corrosionand oxidation resistance. In general, the depth of diffusion obtained isa function of processing parameters and the geometry of the part. Testsfor various mechanical properties of the diffusion coated turbine bladespecimens indicated no deleterious effects.

Although the invention has been described in terms of certain specificembodiments, it is to be understood that other forms may be adoptedwithin the scope of the invention.

What is claimed is: 1. An oxidation and sulfidation resistant articlefor use in a high-temperature environment,

said article being formed of an alloy selected from the group consistingof a nickel based alloy and a cobalt based alloy,

said nickel based alloy consisting essentially on a weight basis 40-80percent nickel, 5-20 percent chromium, up to 10 percent molybdenum, upto 5.5 percent titanium, up to 6.5 percent aluminum, up to 3 percentcolumbium, up to 9 percent tantalum, up to 13.5 percent tungsten, up to2 percent hafnium, up to 1 percent rhenium, up to 1.5 percent vanadium,up to 20 percent cobalt, up to 3 percent iron, and up to minor amountsof carbon, boron, zirconium, silicon, and manganese,

said cobalt based alloy consisting essentially on a weight basis 45-70percent cobalt, 15-30 percent chromium, up to 7 percent molybdenum, upto 10 percent tantalum, up to 16% tungsten, up to 16 percent nickel, andup to minor amounts of carbon, manganese, silicon, titanium, boron, andzirconium,

said article having diffused in the surface of at least a portionthereof the combination consisting initially of aluminum and manganeseto form a diffusion layer of about 0.0004 to 0.006 inch in thickness,said aluminum and said manganese being diffusedthroughout said diffusionlayer, the proportion of said aluminum to said manganese in said PO-105OUNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIQN Patent No.

649,226 March 14, 1972 Dated Inventor(s) It is certified that errorappears in the above-identified patent and that said Letters Patent arehereby corrected as shown below:

Column 1,

Column 2,

Column 3,

Column 4,

Column 6,

line 44, after "portions" insert whereby TABLE I, Chromium, No. 3 shouldread line 71, "horus should read hours line 4, after "enriched" insertin line 39, "test" should be tests Signed and sealed this 22nd day ofAugust 1972.

(SEAL) Attest:

EDWARD M.FLETGHER,JR. Attesting Officer ROBERT GOT'ISCHALK Commissionerof Patents

2. Claim 1 wherein said alloy is said nickel based alloy.
 3. Claim 1wherein said article is a turbine bucket, and said alloy is said nickelbased alloy.
 4. Claim 1 wherein said article is a turbine bucket, saidalloy is said nickel based alloy and said layer is about 0.003 inch inthickness and the proportion of said aluminum is about 18 percent.